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Chemical Evolution

Imposing order on the primordial soup

Munich, 06/06/2012

Life began in a chemical soup when molecular precursors spontaneously assembled to form short nucleic acids. But how could these molecules replicate and transmit information in the absence of proteins? With the aid of temperature variations, perhaps.

In the primordial soup fragments of RNA formed spontaneously from precursor subunits. The resulting short base sequences were the ancestors of our hereditary material, DNA. But before they could evolve into molecules that could carry genetic information, their sequences had to be copied or replicated. In living cells, proteins take care of replication, but how could it be done in the complete absence of proteins?

Professor Dieter Braun and his colleagues at the Institute of Biophysics at LMU Munich have now come up with a possible answer. They have developed and tested a plausible scenario for how primitive RNA could have transferred information. They call it the “RNA-based replicator” model, and it starts with segments of transfer RNA (tRNA) from a microorganism belonging to the Archaea.

Temperature variations structure the chaos

In contrast to most previous attempts to solve the problem, the approach chosen by Braun and his team is based on a phenomenon that would be expected to occur in the primordial soup, temperature fluctuation. As in modern oceans, one would anticipate that variations in temperature gradients would be found near zones of volcanic activity on the floors of ancient seas.

Indeed, temperature changes do impose order on pools of RNA fragments. The LMU team was able to show that thermal fluctuations are sufficient to allow complementary RNA fragments to pair up and separate again. At low temperatures, longer RNAs can serve as templates for the assembly of shorter fragments into full-length strands with complementary base sequences. If temperatures rise, the strands separate, and both can act to templates for further rounds of replication.

In this model system, the primordial soup is a kind of alphabet soup. Information transfer can arise only if sequences of letters can be “read” and copied in the correct order. “The RNA-based replicator points to a possible route from the ‘RNA world’ to modern biology,” explains Hubert Krammer, lead author of the study. “Today, tRNAs translate genetic information into proteins. So attaching amino acids might turn the tRNA replicator into a precursor of protein synthesis.” (Physical Review Letters, 4.6.2012) NIM